Minor CAM cleanups

[Warner Losh]

Just a heads up... Here's a couple of minor cam cleanups...

https://reviews.freebsd.org/D13387 -- creates a xpt_path_inq() wrapper
function
https://reviews.freebsd.org/D13388 -- Makes cam_periph_runccb polling safe
and simplifies the dump code because of it.

Both are net wins. If xpt_path_inq works out, I expect to do it for other
common queries since it makes the code a lot easier to read...

Warner